Process for recovering mercaptans



Patented June 10, 1941 ATNT OFFICE 2,245,317 7 raoonss ro'a anoovnnme MERCAPTANS No Drawing.

8 Claims.

This invention deals with a method for producing relatively pure mercaptans from sour hydrocarbon oils containing them.

It is known that mercaptans can be extracted from sour hydrocarbon oils by contacting the latter with strong aqueous solutions of alkali metal hydroxides. This extraction is particularly effective when the aqueous solution contains an efiective amount of a solutizer .for the mercaptans, the function of which is to raise the solvent power of the aqueous solution for free mercaptans.

As a result of the mercaptan extraction, a fat solution is obtained which contains mercaptides, and from this fat solution the mercaptans can be recovered by steam stripping and condensing the overhead vapors. In the stripping operation a lean solution is obtained which may be recirculated to further extract mercaptans from sour hydrocarbon oils.

The overheadvapors from this operation comprise steam and liberated mercaptans, and upon condensation the condensate segregates into two phases, a water phase and an oily crude mercaptan phase. These phases can readily be separated as by decantation, centrifuging, etc.

I have discovered that mercaptans so recovered are always contaminated with varying amounts of hydrocarbon oil, which oil, even though neutral, apparently is at least partially dissolved in the aqueous alkaline solution in the course of the extraction. The content of neutral oil in the crude mercaptan oil is particularly high, when the aqueous solution contains the above-mentioned solutizers in amounts sufficient materially to raise the solvent power of said aqueous solutions for the mercaptans. In such cases, the content of neutral oil in the mercaptan oil caused by the dissolution of hydrocarbon oil in the aqueous solution may be as high as 80% or even higher. When no solutizer is contained in the aqueous solution, the content of neutral oil in the mercaptan oil is usually lower, say of the order of about %-20%, except where mechanical entrainmentof hydrocarbon oil in the extracting solution takes place. Entrainment is likely to occur particularly when using very strong caustic alkaliv solutions and in the presence of alkyl phenols which may be contained in the hydrocarbon oil, and in such cases the neutral oil content of the crude mercaptan oil may be greatly increased. o

A further source of neutral oil in the mercaptan oil is the incidental oxidation of mercaptans, or of alkali metal mercaptides, which may occur Application December 4, 1939, 'Serial No. 307,449

during the operations of handling and storage of the fat aqueous alkali solution, and/or stripping of the mercaptan oil from this solution. The products of such oxidation are believed to be largely neutral organic disulfides or similar sulfur compounds, which, though largely insoluble in aqueous alkali solutions, are more soluble therein than hydrocarbon oils.

It is difficult in general to separate the mercaptans and neutral oil from one another by fractional distillation, because they have substantially the same boiling ranges.

Now I have discovered that mercaptans substantially free from neutral hydrocarbon and sulfur-bearing oils can be obtained by re-extracting the crude mercaptan oil with an aqueous alkali metal hydroxide solution which is free from solutizers, and reliberating mercaptans from the second extract so obtained as by steaming or acidification. In the second extraction, it is desirable to employ an alkali metal hydroxide solution of not too high a concentration, e. g., not above about 20%, and preferably between about 5%-15%. When using relatively strong caustic solutions, such as 30%-40%, the absorbed mercaptides not only tend to dissolve some of the neutral oil, but also there is again the danger of mechanical entrainment which, under many circumstances, is difficult to avoid.

If it is desired to recover from the crude mercaptan oil substantially all of the mercaptans contained therein, using a batch process for the extraction, it may be necessary to employ an appreciable excess of alkali metal hydroxide over the theoretical amount required to convert all of the mercaptans to mercaptides. The minimum excess varies with the concentration of the aqueous alkali metal hydroxide solution, the neutral oil content of the crude mercaptan oil and the desired mercaptan recovery. For example, when extracting mercaptan oil'containing about 15% neutral oil with a 13% aqueous caustic soda solution, an excess of about 20% over the theoretical amount, applied in three batches, resulted in the recovery of about 99% of the mercaptans. By using a continuous countercurrent extraction system, however, the excess of alkali metal hydroxide, required can be reduced to a much smaller, value.

In the extraction of the mercaptan oil, any

suitable extraction method may be'employed. Thus, the aqueous caustic alkali may be addedin batches with or Without intermediate separation of the oily and aqueous layers until no further decrease in the volume of the oily layeroccur s,

or conversely, no further increase in the volume of the aqueous layer takes place; or the crude mercaptan oil may be subjected to crossor countercurrent extractions with a predetermined amount of the alkaline extracting solution.

The resulting mercaptide solution is then steam stripped or acidified, whereby the mercaptans are liberated in relatively pure form normally containing less than and frequently less than 1%, neutral oil.

Steam stripping has the obvious advantage over acidification of producing a regenerated caustic alkali solution which can be used over for treating further amounts of crude mercaptan oil. On the other hand, where acids stronger than mercaptans, such as alkyl phenols or carboxylic acids, are contained in the crude mercaptan oil, the steam-regenerated solutizer would build up at least a certain amount of these stronger acids, which in turn would cause dissolution of neutral oil in the aqueous solution. This, as pointed out above, is highly undesirable. In such a case, acidification may be preferable.

Acidification with a relatively strong acid, such as sulfuric or hydrochloric acid, will of course liberate not only mercaptans, but alkyl phenols and carboxylic acids as well. CO2 will liberate alkyl phenols, but not carboxylic acids. If the mercaptans are relatively low boiling, they can at least in part be separated from the higher boiling stronger acids by distillation.

The separation between mercaptans and alkyl phenols may be effected by using for the acidification alkyl phenols. In this case, an amount of alkyl phenols should be added to the mercaptide solution which is just sufiicient, but no more than required to bind substantially all of the alkali metal in the aqueous solution. Mercaptans are liberated and may be separated from the resulting alkyl phenolate solution, and the latter may then be acidified with CO2 to liberate the alkyl phenols. Alkyl phenols so recovered are usually considerably contaminated with mercaptans, and at least a portion of them is advantageously employed to liberate mercaptans in the manner disclosed above.

It is desirable to exclude air or other oxidizing media from the fat alkaline extracting solution, since it is found that alkaline solutions of mercaptans are particularly susceptible to oxidation causing formation of organic disulfides or related non-acidic sulfur compounds at the expense of the mercaptans. Thus it is desirable to steam or acidify the alkaline solution in the absence of oxidizing agents, such as air, etc., and to carry out the liberation of the mercaptans as soon as possible, e. g., immediately after the alkaline extracting solution has been charged with the mercaptans from the crude mercaptan oil, so that the danger of access of air is minimized. Formation of neutral oil by conversion of mercaptans, tending to defeat the purpose of my invention, is thus avoided.

In the foregoing description, solutizers for mercaptans have been mentioned repeatedly. Solutizers have been defined as organic compounds which are readily soluble in the aqueous alkaline extracting solution, are substantially insoluble in hydrocarbon oil, are good solvents for the free mercaptans, are inert to the action of caustic alkali even at the elevated temperatures of steam stripping, and preferably have boiling temperatures substantially higher than that of water. Among, the many substances having solutizing properties, the following have been found to be most useful: Primary, secondary and tertiary amino or hydroxy alkyl amines in which the alkyl radical has from 2 to 3 carbon atoms; glycols, amino glycols and diamino alcohols of 3 to 5 carbon atoms; diamino, dihydroxy or amino hydroxy dialkyl ethers or thioethers in which the alkyl radicals have 2 or 3 carbon atoms; alkali metal, and particularly potassium salts of fatty acids having from 3 to 5 carbon atoms, or of mono hydroxy or amino fatty acids having from 4 to 7 carbon atoms, or of phenyl acetic, mono amino or hydroxy phenyl acetic acids, or of alkyl phenols; or mixtures of the above.

The following examples serve to illustrate my invention:

Ewample I A light cracked gasoline fraction of 210 F. end point, distilled from pressure distillate, and containing 0.20% mercaptan sulfur, was extracted with a 14 B. caustic soda solution to produce an extracted distillate containing 0.03% mercaptan sulfur. The spent soda solution containing 3.2% mercaptide sulfur was stripped with steam, and the resulting foul overhead vapors were condensed and allowed to separate into two layers. The upper liquid layer comprising the mercaptans was decanted and recovered. It had a gravity of 40.3 A. P. I.

995 gm. of this crude mercaptan oil was extracted with three batches of 12.8% aqueous NaOI-I, the first two batches being 2,000 ml. each and the third 200 ml. This amount of caustic soda represented an excess of about 18% over the theoretical amount to convert all of the acidic material to salts. 13% by weight of the crude mercaptan oil remained undissolved. This residue had an A. P. I. gravity of 714.

The mercaptide solution resulting from this last treatment was separated from the residue and was then acidified. 840 gm. of mercaptans substantially free from neutral oils were obtained.

Example II A pressure distillate fraction having a mercaptan sulfur content of 0.15% was extracted with a 16 B. aqueous caustic soda solution to reduce its mercaptan sulfur content to 0.05%. The resulting spent caustic liquor was steam stripped and an overhead crude mercaptan oil was obtained consisting of mercaptans, in addition to neutral oils and alkyl phenols.

1740 ml. of this crude mercaptan oil was extracted with 4980 m1. of a 4.7% aqueous NaOH solution. 460 ml. or 32.6% by weight of the mercaptan oil dissolved in the aqueous solution. The resulting aqueous and oily layers were separated.

Upon acidification of the aqueous layer, an oily liquid was obtained which was redistilled, yielding 402 m1. of a distillate. This distillate had the following approximate composition:

Per cent Methyl mercaptan 6 Ethyl mercaptan 40 Isopropyl mercaptan 25 Tertiary butyl mercaptan 6 n-Propyl mercaptan 5 Higher boiling mercaptan 18 Total I claim as my invention: 1. In the process of producing relatively pure mercaptans from a hydrocarbon oil containing them, the steps comprising extracting said bydrocarbon oil with first a strong aqueous alkali metal hydroxide solution to produce a treated hydrocarbon oil and a fat aqueous solution containing mercaptides and neutral oil, separating the fat solution from the treated oil, stripping the former with steam to vaporize mercaptans and neutral oils, condensing the resulting vapors to produce a condensate which segregates into two phases, a water phase and a crude mercaptan oil phase, separating the phases, treating the crude merc-aptan oil with a second aqueous solution of an alkali metal hydroxide having a concentration not above 20% to dissolve mercaptans contained in the mercaptan oil, thereby producing a mercaptide solution and a residual oil insoluble in said mercaptide solution, separating said mercaptide solution from the residual oil, and reliberating mercaptans from said mercaptide solution.

2. The process of claim 1 in which the first aqueous alkali metal hydroxide solution contains a solutizer for mercaptans in an amount sufficient materially to raise the solvent power of said solution for mercaptans.

3. The process of claim 1 in which the concentration of the second aqueous solution of alkali metal hydroxide is between 5% and 15%.

4. The process of claim 1 in which the reliberation of the mercaptan from the mercaptide solution is effected by steam stripping.

5. The process of claim 1 in which the reliberation of the mercaptans from the mercaptide solution is efiected by acidification with an acid having a greater dissociation constant than mercaptans.

6. The process of claim 1 in which the reliberation of the mercaptans from the mercaptide solution is efiected by acidification with alkyl phenols.

7. In the process of producing relatively pure mercaptans from a hydrocarbon oil containing them and alkyl phenols, the steps comprising extracting said hydrocarbon oil with first a strong aqueous solution of an alkali metal hydroxide to produce a treated hydrocarbon oil and a fat aqueous solution containing mercaptides, alkyl phenolates and neutral oil, separating the fat solution from the treated oil, stripping the former with steam to vaporize the mercaptans, alkyl phenols and neutral oil, condensing the resulting vapors to produce a condensate which segregates into two phases, a water phase and a crude mercaptan oil phase, separating the phases, treating the crude mercaptan oil with a second aqueous solution of an alkali metal hydroxide, having a concentration not above 20% to dissolve mercaptans and alkyl phenols contained in the crude mercaptan oil, thereby producing a mercaptide solution containing alkyl phenolates and a residual oil insoluble in said mercaptide solution, separating said mercaptide solution from said residual oil, adding to the separated mercaptide solution an amount of alkyl phenols just sufiicient to bind substantially all of the alkali metal in said mercaptide solution. thereby liberating mercaptan and producing an alkyl phenolate solution, separating the liberated mercaptans from the alkyl phenolate solution, liberating alkyl phenols from the latter and using the resulting liberated alkyl phenols again to liberate mercaptans from said mercaptide solution. 4

8. The process of claim '7 in which the alkyl phenols are liberated from said alkyl phenolate solution by acidification with CO2.

RICHARD A. BANNEROT. 

